Accurate measuring of the thickness of the refractory wall of a furnace vessel and detecting penetrant metal in the refractories are extremely important inspection items in the safety control and maintenance work of the refractories. A measuring apparatus using electromagnetic induction sensors on high-frequency current for obtaining refractory lining profiles of such a furnace vessel is disclosed in JPA-2-025687 by the present applicant, but this technique has a drawback that if a large amount of penetrant metal exists in the lining refractories, the electromagnetic waves are intercepted by the metal, and the refractory thickness cannot be measured accurately.
A method of automatically measuring the remaining lining thickness is disclosed in JPA 51-147510, for example, which uses waves such as laser light waves or microwaves to obtain the distance between the refractory wall and the wave generator based on the time duration or the phase difference of waves from the emission by the wave generator till the return to the wave generator after being reflected by the refractory wall, and based on changes of this distance, calculates the position and the amount of corrosion and the thickness of the refractory wall. However, this method is not used very often because of the inability to constantly fix the relation between the installed positions of the measuring apparatus and the vessel, and also because of a frequent occurrence of errors in measurement affected by dust or the fluctuation of the light derived from the long distance between the measuring point and the light source. Furthermore, the apparatus of this prior art, being designed for surface inspection, is unable to obtain information as to the penetrant metal.
JPA 62-34003 discloses an eddy current type electromagnetic sensor for detecting a conductive substance, that is, metal existing in the refractories. This sensor called NS Nippon Steel Metal sensor for detecting such a penetrant metal comprises two coils made by winding nonmagnetic conductors, that is to say, a wave transmitting coil and a wave receiving coil having a space between them and disposed at positions symmetric with respect to the axis of symmetry. The sensor is disposed to face the surface of the metal in a manner that the axis of symmetry is within ten degrees from the normal line to the surface of the metal.
When a high frequency a.c. current is conducted through the wave transmitting coil, a high frequency magnetic field (primary magnetic field) is produced, an eddy current is induced in an electric conductor to be detected, and a secondary magnetic field is produced by this eddy current. An induced voltage is generated in the wave receiving coil by a composite magnetic field made by the primary magnetic field and the secondary magnetic field, and this induced voltage is determined by the distance between the sensor head and the electric conductor. Therefore, when there is no metal in the refractories, the sensor detects the vessel shell or iron surface behind the refractories, and from this output voltage, the thickness of the refractories can be found.
If metal has intruded between the vessel shell and the sensor, an eddy current is produced in the surface of the metal. As the metal is closer to the sensor than the vessel shell, a greater output voltage is produced than the output voltage corresponding to the distance between the sensor and vessel shell when nothing other than the refractories exists between the sensor and the vessel shell. In this case, this output voltage is in a magnitude corresponding to the area and the depth of the metal.
In this way, metal existing in the refractories can be detected and measured, and on the basis of measurement results, the vessel can be overhauled or repaired before metal leakage occurs.
However, the above-mentioned sensor cannot measure the refractory thickness where there exists penetrant metal.
As described above, the conventional methods and apparatus cannot acquire accurate measurements of the refractory thickness and detect penetrant metal.